Geochronology and Geodynamic Settings of Zenong Group Tuff from Shiquanhe of Middle Lhasa Terrane, Tibet
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摘要: 前人对西藏拉萨地体则弄群的时代仍存在晚侏罗世与早白垩世的争议，且缺乏东西方向上的对比研究.对位于西段的狮泉河则弄群凝灰岩进行了系统的锆石U-Pb年代学、锆石微量元素和Hf同位素研究，完善了则弄群的年代学格架.狮泉河则弄群凝灰岩的锆石多为自形长柱状，发育明显的振荡环带，其微量元素特征显示为岩浆锆石.2件凝灰岩样品LA-ICP-MS锆石U-Pb测年结果显示其成岩年龄分别为157.5±0.9 Ma和159.2±1.3 Ma.结合前人研究成果，确定则弄群时代为晚侏罗-早白垩世.凝灰岩的锆石微量元素与Hf同位素特征表明其形成于大陆弧环境，源区可能由古老的陆壳基底与少量幔源物质混合形成.区域资料表明，Slainajap洋盆在东西延伸方向上俯冲起始时间不同，可能存在西早东晚的特征.Abstract: The formation age of the Zenong Group along the Shiquanhe-Yongzhu-Namucuo-Jiani mélange in the middle Lhasa terrane remains controversial between the Late Jurassic and the Early Cretaceous, and there is a lack of comparative study along the east-west direction. In this study, it focuses on the Zenong Group tuff from the Shiquanhe area in the western segment, and systematic analyses of zircon U-Pb age, trace element and Hf isotope for zircon were done for the tuff samples. Zircons from the tuff samples are elongated grains, showing obvious oscillatory zones, and the trace elements also verify the magmatic origin of zircons. Two tuff samples show LA-ICP-MS zircon U-Pb ages as 157.5±0.9 Ma and 159.2±1.3 Ma. Combined with previous research results, the Zenong Group was formed in the Late Jurassic to Early Cretaceous. Zircon trace elements and Hf isotopes of the tuff samples indicate that the Zenong Group were formed in the continental arc setting, and the hybrid of ancient continent crustal basement material and minor mantel material might be the magma source. Combined with the previous studies, the initial time of the subduction might vary from the earlier one in the western part to the later one in the eastern part.
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Key words:
- Shiquanhe /
- Zenong Group /
- tuff /
- geochronology /
- Slainajap Ocean /
- Lhasa Terrane
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图 1 拉萨地体中生代岩浆岩分布
底图据邹洁琼等（2018）修改.狮泉河蛇绿岩年龄据Xu et al.(2014)；则弄群火山岩年龄据康志强等（2008）、朱弟成等（2008）、刘伟等（2010a）、周华等（2016）、王力圆等（2016）和叶春林等(2018). BNS.班公湖-怒江缝合带；IYZS.雅鲁藏布江缝合带；SNMZ.狮泉河-永珠-纳木错-嘉黎蛇绿混杂岩带；LMF.洛巴堆-米拉山断裂带；STDS.藏南拆离断层系；MBT.主边界逆冲断层；MCT.主中央逆冲断层
Fig. 1. Distribution of the Mesozoic magmatic rocks in the Lhasa Terrane
图 2 西藏狮泉河区域地质简图
西藏自治区地质调查院，2015. 1:5万西藏噶尔地区6幅区域地质调查报告
Fig. 2. Simplified geological map of Shiquanhe area, Tibet
图 3 狮泉河则弄群凝灰岩野外露头与镜下特征
J3K1Z.则弄群；K1l.郎山组；Q.石英；Pl.斜长石；Bt.黑云母；Ser.绢云母；ANe.安山岩岩屑
Fig. 3. The outcrop and microphotographs of the Zenong Group tuff from Shiquanhe area
图 4 狮泉河则弄群凝灰岩样品锆石阴极发光图像
实线圈代表锆石U-Pb同位素测试点位，虚线圈代表锆石Hf同位素测试点位；数字代表点号
Fig. 4. Zircon CL images for the Zenong Group tuff from Shiquanhe area
图 5 狮泉河则弄群凝灰岩样品锆石U-Pb年龄谐和图
Fig. 5. Zircon U-Pb concordia diagrams of the Zenong Group tuff from Shiquanhe area
图 6 狮泉河则弄群凝灰岩样品锆石稀土元素球粒陨石标准化配分图解
标准化数据据Sun and McDonough (1989)
Fig. 6. The chondrite-normalized REE patterns for the Zenong Group tuff from Shiquanhe area
图 7 狮泉河则弄群凝灰岩锆石Nb-Ta（a）、Th-Y（b）、Th-U（c）与Hf-Y（d）图解
Fig. 7. Nb vs. Ta(a), Th vs. Y(b), Th vs. U(c) and Hf vs. Y(d)diagrams of zircons from Zenong Group tuff in Shiquanhe area
图 8 中拉萨地块、北拉萨地块中侏罗-早白垩世早期岩浆活动的锆石εHf(t)-t图解
Fig. 8. Zircon εHf(t) vs. t plot for the Middle Jurassic-Early Cretaceous magmatic activity in central and northern Lhasa Terrane
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